1. Field of the Invention
This invention relates to an over-head projection with a partial projection device, and more particularly to an over-head projector capable of projecting a part of an original image in enlarged scale.
2. Description of the Prior Art
There have been known two types of partial projection device for an over-head projector, one of which employs a relay lens between the original image on the fixed stage and the projection lens for changing the field of view of the projection lens system, the other employing a platform located above the stage for holding the original image at an elevated level above the stage and a converter lens combined with the projection lens for changing the effective focal length thereof.
The former type of the above-mentioned over-head projector with the relay lens is disadvantageous in that the brightness of the image projected is lowered since the original image is located on the stage and the small area thereof illuminated by the same light source is projected in enlarged scale.
The latter type of the over-head projector with the converter lens is advantageous in that the brightness of the image projected in enlarged scale is not so much lowered. However, this type of projector is disadvantageous in that it requires troublesome operations such as attaching and detaching of the converter lens and the platform.